Pharmacokinetics, biodistribution and brain retention of a bispecific antibody-based PET radioligand for imaging of amyloid-ß.

Monoclonal antibodies (mAbs) have not been used as positron emission tomography (PET) ligands for in vivo imaging of the brain because of their limited passage across the blood-brain barrier (BBB). However, due to their high affinity and specificity, mAbs may be an attractive option for brain PET if their brain distribution can be facilitated. In the present study, a F(ab')2 fragment of the amyloid-beta (Aß) protofibril selective mAb158 was chemically conjugated to the transferrin receptor (TfR) antibody 8D3 to enable TfR mediated transcytosis across the BBB. The generated bispecific protein, 8D3-F(ab')2-h158, was subsequently radiolabeled and used for microPET imaging of Aß pathology in two mouse models of AD. [124I]8D3-F(ab')2-h158 was distributed across the BBB several fold more than unmodified mAbs in general and its accumulation in the brain reflected disease progression, while its concentration in blood and other organs remained stable across all age groups studied. Cerebellum was largely devoid of 8D3-F(ab')2-h158 in young and middle aged mice, while mice older than 18 months also showed some accumulation in cerebellum. In a longer perspective, the use of bispecific antibodies as PET ligands may enable in vivo 'immunohistochemistry' also of other proteins in the brain for which PET radioligands are lacking.